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(Updated 25-April '07)

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Mini Garden Tractor

Introduction     Layout     Structure     Drive     Performance     Free Plans     Videos

The mini garden tractor is one of my earlier designs and I now have a newer 1300W Yard tractor design available - click on the small image on the right to go to its web page.

The new design is a more robust vehicle with a bigger drawbar pull.

The Earlier Design

Tractor Front

This early mini tractor was a different beast from its predecessors. Designed less for fun and more for practical use around the garden. It is fun to use however although its lack of protective structure around the wheels means it can't be run into walls etc without damaging consequence: it needs to be driven with more care. It has a standard tractor form with large rear wheels for traction and smaller front wheels for steering. A draw bar is built in low and close to the rear axle and a simple and effective trailer provided for practical use. Overall design aims were to produce a compact vehicle with useful tractive capability for garden use. Ease of access for the user and good manoeuvrability were also aims.

 Tractor & TrailerThe vehicle width and length have been constrained to achieve compactness necessary for storage and manoeuvrability. Structure does not protrude beyond the wheel envelopes so allowing the tractor to be manoeuvred in tight spaces. The seating position is above and forward of the rear axle to contribute to the COG position which gives stability and required weight over axle for effective traction. Clear access for the driver to and from the seating position also results. Steering is by hand tiller and speed control by foot pedal. The two 40Ah 12v batteries are set low between the front and rear axles easily accessible for charging or removal. The drive components are set above and between the rear wheels. The tow bar position is set to ensure ease of trailer engagement/disengagement and effective transfer of haulage forces into the structure.

View From Back The structure is of ply and planed soft wood with a screwed and glued construction much like Buggy No.2. There are no welded structures. In this vehicle all wheels are cantilevered from their support structures and through hardened steel axles have been used all round to carry the increased bending moments. The axles are supported by deep groove ball bearings housed in the vehicle's wooden structure. The front wheels are carried on a wooden beam axle pinned to the main vehicle structure to provide articulation necessary to keep the front wheels in ground contact on uneven ground.

The steering geometry is modified ackerman to reduce tyre scrub on cornering. The front wheel support structures have been the source of some redesign work; some elements made from wood would probably better be made in steel to carry the considerable local forces that exist on turning and impact. A hand tiller provides the steering actuation and is effective in giving adequate sensitivity of control and is an improvement over original plans for the foot steering system shown in some of the images.

 TractorThe rear drive wheels are doubled-up mountain bike wheels. This enhances load bearing capacity and produces a stiffer wheel assembly better capable of resisting lateral loading. It also improves traction over that provided by single bike wheels. Lateral stiffness of the wheels is significant because of the drive train design and its need for reasonable running alignment of the drive rims attached to the wheels. The front wheels are also bike wheels but smaller, in this case plastic moulded BMX wheels. All wheels have had their existing ball races stripped and replaced with static connections to the rotating axles.

The unladen tractor weighs about 80kgf and the trailer 26kgf. With driver and 100kgf load the all up mass is about 280kg.

 The core drive components; the motors, controller and batteries have been inherited from the previous vehicles. Each rear wheel assembly is driven by two permanently engaged DC PM motors through 2 stage speed reductions of about 36:1. Expensive or difficult to obtain reduction gearing has been avoided by using a combination of synchronous belt and sprocket on chain drives. The large reduction results in sufficient drive torque to haul the loaded trailer (and other loads) but has reduced the top speed to about 12 km/h. This is fine for garden use. Adhesive bonding of drive components onto shafts has again been employed to ease manufacture. These bonds have sustained full torque loading through the drive train including full stall conditions for the motors. The bonding may give problems in the future when parts need to be changed though!

 The 4QD controller gives sensitive speed control in both forward and reverse. No mechanical brakes have been fitted, mainly because our garden is pretty level with only local steep gradients and the motors and controller effectively decelerate and stop the vehicle. A parking brake might be considered to stop vehicle creep when loaded and stationary on an incline.

The tractor hauls well, the limit so far is that of traction; the wheels usually spin before the motors stall. Manoeuvrability is good, although the younger drivers have discovered that reversing a tractor and trailer isn't quite as easy as it looks. Battery life isn't much of an issue as it's use in the garden is for mainly intermittent duties. I haven't been able to gauge this properly yet and the batteries are on their 3rd vehicle and aren't what they once were.

The principal noise from the tractor is the characteristic “buzz” from the chain drive rims... it's still much quieter than an IC powered vehicle though.

CAD Rendered TractorProblems have mainly been those related to stressed joints in timber ie pin and bearing housings. These tend to open out slightly under the action of the compressive contact stresses from the steel pins or bearings inducing some unwanted movement in the wheels or steering. This has been resolved by inserting packing but should be addressed by reinforcement of the housing holes. An initial front wheel support structure configuration failed due to over stressing on some wooden parts and was redesigned.

We didn't get around to painting the tractor, if we had it probably would have ended up looking like the CAD rendering shown just above.

Engineering drawings of the tractor are available as a free downloadable sheets.....have a look at the tractor plans page.

Here are three short .AVI video clips showing the tractor running shortly after building - there's no cover or proper seat on yet!

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